Answer: We Have two kidneys as we need some extra reserve of kidney mass in case we sustain some sort of injury.
A kidney is made up of nephrons ,which is its job to filter,reabsorbs solutes and water.
More kidney mass,more nephrons.
Your kidneys are important they help your bones create new blood cells and have many important functions.Some people are only born with one but you still can survive ,having two is just something like a replacement if the kidney fails(shuts downs) due to an injury.